This question was posted in November of last year, and remains unanswered.

Out of curiosity in Australia the word "Fanny" or as it is also colloquially termed "the map of Tassie" (Tasmania being reminiscent in shape) refers to that part of female anatomy or 'front bottom' as Robert kindly touched on, but in the US of A "fanny" means bum. Can someone elaborate on when or how this difference evolved?
brian - Australia

Fanny refers to the same area in the UK as well, and after a glorious mix up with a visiting American lady when she told me that putting the covers on a waterbed was like patting a big fanny, and I told her I would knock her up in the morning before I went to work (I would knock her bedroom door to make sure that she was awake), I have long wondered why such a word or phrase can be innocent in one country and not be used in mixed company in another.

The Cassell Dictionary of Slang states 'etymology unknown' for this word, although it does comment that "Eric Partridge suggests a link to Fanny Hill, the heroine of John Cleland's Memoirs of a Woman of Pleasure (1749)."

As a name, Fanny is a diminutive of Frances, whose meaning is 'Frankish, French', but this fact does not seem to point in any obvious direction, etymologically speaking.

sorry about this first post from me, I'm new here and not sure about the correct way to do things but I did want to say in response to the fanny post...

(I think this is an hilarious post...
I need to remember to knock you up)
me

(and might I add,.... bum in the US of A means 'no-good-beggar' or tramp???
and in the UK,... your bum means your buns.... which in the US of A means your Butt Cheeks,
and buns in the UK means what wraps Hamburgers in ??? I think,... er,... yeh?? does it??)
and buns in the US of A also means Hamburger Wrappings also?? ... the round softball sized bread sliced
once through the horizontal middle into which a meat patty is inserted to create a hamburger..

There are quite a number that I know. Someone dying for a fag in the UK merely wants a cigarette, whilst a faggot is either a bundle of sticks (in fairness hardly ever used nowadays), or a savoury meat ball served in gravy.

It even works in gestures, If an American extends his middle finger and waves it around vertically it apparently means something rude, whereas if a Brit were to make his index and middle fingers into a ‘V’ shape he knows he is being extremely rude, but there is a fair bet that he doesn’t know why it is actually. I know of two theories, either of which sounds plausible. In truth I think the American gesture is probably older than the British "V" sign, it is certainly more obviously phallic.

Thinking about it, perhaps I should have started this thread in Usage.

Brewer’s Dictionary of Phrase & Fable tells us, as Erik mentioned above, that some have traced the FANNY back to Fanny Hill, the libidinous heroine of John Cleland’s Memoirs of a Women of Pleasure (1748-9). They say her own name 'Fanny,' the diminutive of the feminine name Frances, which is often used as a short title of the book, is apparently a pun on the anatomical term ‘mons veneris’ (the mons pubis of the human female – from New Latin, literally Venus’s mount) – I guess I don’t get that pun! How do we go from ‘mons veneris’ to 'Fanny'? (another more easily understood – at least to me – pun theory is that the front ‘fanny’ came from the obsolete expression ‘fancy vulva’).

The first use of FANNY in the sense of ‘female genitals’ was in 1835-40 (see quote below). Brewer’s erroneously said late 19th century and the Oxford Dictionary of Slang said 1879 (see quote below), but in any case there is a sizeable gap between the publication of the book and the first used of ‘fanny’ in this sense, which does raise doubt about any possible connection.

The first documented use of FANNY in the U.S. as backside / buttocks / ass / 'human rump' was in 1919 (see quote below) and the sense of the female front never did make it over here. How FANNY went from frontside to backside, or if there is even a connection between the two words, is unknown. But two conjectures that have been suggested are: 1) it is a pun on the word ‘fundament’ (1297, the buttocks), which first became ‘fun,’ then ‘fan,’ and then ‘fanny’ 2) Webster’s derives it from “the fanciful euphemism ‘Aunt Fanny’” for the buttocks.

It is interesting to note that Fanny Hill remained on the banned book list in the U.S. until 1966 under the Comstock Law of 1873 (named for Anthony Comstock, who led the Society for the Suppression of Vice), officially known as the Federal Anti-Obscenity Act. This law banned the mailing of 'lewd,' 'indecent,' 'filthy,' or 'obscene' materials. Earlier in 1821 Fanny Hill was banned in Massachusetts, leading to what is billed as the first obscenity trial in the U.S. in which the publisher, Pete Holmes, was convicted for printing a ‘lewd and obscene’ novel.

<1919 “They made us all get in a circle and stoop over while a guy ran around and hit us on the—never mind where—with a strap—I believe they call the game “Bat the FANNY” and they sure did bat me.”—‘12th U.S. Infantry.’ page 73>

<1925 “They can . . . hang them on the Commanding General’s FANNY [[l.c.]] for all I care.”—‘Manhattan Transfer’ by Dos Passos,’ page 273>